The prior art has disclosed hydrostatic transmissions for mobile working machines in which a hydrostatic pump (primary unit) and at least one hydrostatic motor (secondary unit) are fluidically connected to one another by means of a closed hydraulic circuit. A primary unit may be situated with multiple secondary units in a closed hydraulic circuit, wherein the secondary units are arranged in parallel with respect to one another in terms of the circuit configuration. An internal combustion engine, for example a diesel engine of the mobile working machine, is coupled rotationally conjointly to the primary unit, and an output, for example an axle or a wheel of the mobile working machine, is coupled rotationally conjointly to the secondary unit. The mobile working machine thus has a traction drive which has a hydrostatic transmission.
The document EP 1 960 699 B1 discloses a hydrostatic transmission of said type, with which braking can also be performed. Here, in relation to traction operation, the power flows in the reversed direction from the output, via the secondary unit acting as a pump and via the primary unit acting as a motor, to the internal combustion engine, which is then driven in a passive cranking operating mode. The highly pressurized working line of the closed circuit is in this case safeguarded by means of a pressure-limiting valve, by means of which, too, a part of the braking power can be dissipated during the braking operation.
A problem during such braking operations is that the internal combustion engine may be driven only with a maximum rotational speed in order that it is not destroyed. Therefore, the primary unit, which is designed as an adjustable axial piston machine and which operates as a motor, is, during the braking operation, set to a pivot angle at which a torque is generated that can still be supported by the internal combustion engine without the latter being caused to overspeed. This occurs taking into consideration the pressure determined in the working line by the setting of the pressure-limiting valve.
The documents DE 10 2014 211 393 A1 and DE 10 2014 211 394 A1 each likewise disclose a hydrostatic transmission with which braking can also be performed, wherein it is sought to protect the internal combustion engine against overspeeding. A first part of the braking power is output via the primary unit to the internal combustion engine, whereas a second part of the braking power is converted by means of the pressure-limiting valve of the high-pressure line in question into heat. Here, a rotational speed of the internal combustion engine which is initially slightly higher than the admissible rotational speed limit is accepted.
The respective primary units of the stated prior art are subjected to closed-loop pivot angle control. For this purpose, they require feedback of their present pivot angle. Furthermore, in the context of the pilot control of the pivot angle, a pivot angle that is generally higher than the ideal pivot angle is disclosed.
By contrast to this, the disclosure is based on the object of providing a hydrostatic transmission and a method, in which the pivot angle and thus the swept volume (during braking operation, this is a displacement volume) of the primary unit are optimally controlled, or controlled in closed-loop fashion, such that the rotational speed of the internal combustion engine is brought to its maximum rotational speed as quickly as possible and is held as close as possible to this maximum rotational speed. Here, the internal combustion engine should not exceed its maximum rotational speed at any time.
Furthermore, the outlay in terms of apparatus for the primary unit should be reduced.
Said objects are achieved, with regard to the hydrostatic transmission, by means of the combination of features disclosed herein and, with regard to the method, by means of the combination of features disclosed herein.